Animals employ conserved sets of patterning molecules during development. Differences in animal form arise from the differential regulation of these conserved molecules and their downstream targets in different evolutionary lineages. This proposal aims to identify the mechanisms by which homologous fields are elaborated into diverse morphologies, using the insect wing as a model of morphological diversity. The Drosophila wing and haltere are two morphologically distinct structures derived from common phylogenetic and morphological precursors. One striking difference between the wing and haltere is a difference in cell number-the wing disc contains 50,000 cells, whereas the haltere disc contains only 10,000 cells. Therefore, regulation of growth control must play an important role in establishing the morphological difference between wing and haltere. Microarray analysis will be used to identify genes that are differentially regulated between the wing and haltere. Genes identified in this analysis will be tested for direct regulation by Ultrabithorax (Ubx), a homeobox protein that regulates the identity of the third thoracic segment in insects. The cis-regulatory elements that confer regulation by Ubx will be identified and will be functionally compared to cis-regulatory elements isolated from the same genes in different insect orders. This project offers a general model of how regulatory networks can be modified and how this modification impacts morphology.